1. Field of the Invention
The present invention relates to a device for power factor correction for connection to an AC voltage network, comprising a transformer with, on the one hand, high-voltage terminals for connection to the network and, on the other, low-voltage terminals, a capacitor battery for connection to the network, and a controllable inductive element connected to the low-voltage terminal of the transformer.
2. The Prior Art
FIG. 1a shows a known arrangement of this type. To an AC voltage network 1 there is connected a capacitor battery C. To the network there is also connected a reactor R by way of a transformer TR. The current through the reactor can be controlled continuously between zero and the rated current by phase angle control of the anti-parallel-connected thyristors 2 and 3 connected in series with the reactor. The transformer steps down the mains voltage to a lower working level suitable for the thyristors and the reactor. Arrangements of this type are used for compensating the reactive power consumed by load objects B connected to the network 1. The capacitor C generates reactive power, and this reduces the total reactive power taken from the network. The reactive power consumption of the load object frequently varies, and it is then desirable that the reactive power generated by the power factor correcting means should correspond at each moment as closely as possible to the reactive power consumed by the load object.
This is achieved in the device according to FIG. 1a. Its mode of operation is shown in FIG. 1b. The reactive power consumption of the load object is expected to vary between zero and a maximum value Q.sub.max. If full compensation is to be obtained, at each moment the rated powers (rated voltage x rated current) of the capacitor battery and the reactors are chosen to be equal to Q.sub.max. FIG. 1b shows the reactive powers, Q.sub.C and Q.sub.R, respectively, of the capacitor battery and the reactor as functions of the total inductive power Q.sub.T generated by the correcting device. The reactive power Q.sub.TR flowing through the transformer is of course equal to Q.sub.R, and the rated power of the transformer is Q.sub.max. In general Q.sub.T will be lower than Q.sub.max, in certain typical modes of operation (such as correction of arc furnaces) essentially lower. As a rule, therefore, a considerable amount of power (Q.sub.TR) flows through the transformer, the losses of which will therefore be relatively high.